1. Field of the Invention
This invention relates to semiconductor wafer transfer apparatus and, more particularly, to transfer apparatus for moving semiconductor wafers vertically and horizontally to transfer the wafers from one location or station to another.
2. Description of the Prior Art
Semiconductor discs or wafers, made of silicon and other materials, are the primary ingredient of semiconductor chips. Many, many chips can be made from a single wafer. During the manufacturing of integrated circuits, the wafers are processed numerous times. The wafers accordingly must be transferred between containers or holding elements numerous times for the various manufacturing steps.
The containers or holders are typically referred to in the industry as "boats". There are primarily two kinds of boats. One kind of boat is a quartz boat. It is used to hold the silicon wafers when the wafers are processed in ovens. For other processing, such as when chemical baths are used to treat the wafers, plastic boats are used.
For purposes of quality control, it is preferable to have the silicon wafers transferred automatically, without the intervention of human hands. Each time a wafer is touched by a human hand, even a gloved hand, there is danger of both breakage and contamination. As will be understood, the silicon wafers are relatively thin and fragile, and thus are susceptible to breakage. It is accordingly desirable that the wafers be transferred from quartz boats to plastic boats, and vice-versa, by automated machinery.
Typically, the silicon wafers in boats are quite close to each other. A typical quartz boat will hold fifty wafers. The quartz boats hold the wafers primarily for heating purposes in ovens, and accordingly there does not need to be much space between the wafers. The spacing in plastic boats is about half the density, or about twice as great as the spacing of the wafers in the quartz boats. There are normally about twenty-five wafers per plastic boat. The reason for the difference in spacing is that the wafers in the plastic boats will be subject to acid baths or other chemical processing, etc., and thus greater spacing is required between wafers.
The apparatus of the present invention is concerned with the transfer of the wafers from quartz boats to plastic boats, and from plastic boats to quartz boats. Since there are twice as many wafers in a quartz boat as in a plastic boat, one quartz boat requires two plastic boats to hold the same total number of wafers. The apparatus of the present invention has three stations, one station for a single quartz boat, and two stations for plastic boats.
U.S. Pat. No. 2,083,618 discloses packaging apparatus in which elements to be packaged are carried or are transported in conveyor elements which include cutout portions for receiving the elements to be transported.
U.S. Pat. No. 3,080,955 discloses a conveyor system in which roller elements are used to move fruit. The roller elements include cutout or relieved portions which assist in moving the fruit.
U.S. Pat. No. 3,830,355 discloses apparatus for longitudinally separating cigarettes which are moved on a conveyor system in an end-to-end relationship. The spacing is accomplished by roller elements which include cutout or relieved portions which contact the cigarettes and accelerate them in single file.
U.S. Pat. No. 3,858,708 discloses another type of material handling apparatus in which rollers are used to move cylindricai or rod-like elements.
U.S. Pat. No. 3,976,499 discloses roller apparatus for conveying and cutting sugar cane stocks. A pair of rollers is shown receiving stocks from a conveyor system. The rollers are not round, rather they are of a somewhat squarish cross-section, with convexly extending corner portions.
The patents discussed above generally refer to materials handling or conveying apparatus which include some type of roller mechanism or elements related thereto. However, none of the apparatus shown in the patents is used to convey wafer-like elements, and none of the apparatus utilize roller elements for holding and positioning purposes. Rather, the roller elements are used for conveying articles.